The present invention relates to an apparatus for improving the efficiency of contact printing vacuum frames and more particularly to a vacuum sheet for use in vacuum frames which reduces the time required to complete vacuum distribution between the blanket and glass portions of the vacuum frame.
The use of contact vacuum frames in the production of film positives and negatives, lithographic plates, peal coats, scribes and colour proofs is of course well known and there are a number of well established manufacturers in the field producing contact vacuum frames of various sizes ranging from a few feet on each side to dimensions of up to 15 feet by 5 feet.
Generally speaking, the purpose of a vacuum frame is to ensure the closest possible contact between original material to be reproduced onto film and the film itself without any intervening air pockets (also called hot spots) between either the original material and the film or between these sandwiched materials and the vacuum frame glass.
The materials to be reproduced are placed between the glass and blanket portions of the frame which are brought together and sealed. A vacuum is then applied to the space between the blanket and the glass to force the sandwiched materials together and against the vacuum frame glass. Typically, negative or vacuum pressures of approximately 25 psi are required with the air being drawn from air outlets provided at the extremes of the frame such as at the ends or corners of the blanket.
A primary drawback of vacuum frames is the length of time required to make a good "contact", that is, the time required for the complete and even distribution of vacuum within the frame to eliminate hot spots. This typically requires anywhere from 10 to 30 minutes or more, depending upon the size of the frame and the number of layers of material sandwiched together, which may be quite a few particularly when preparing colour proofs. This waiting or "vacuum" time is essentially non-productive down time.
After the prolonged waiting period, printers will look for physical indicia of a good contact such as an even distribution of Newton's Rings, an oily looking pattern of squiggles, between the top film and the contact frame glass. The appearance of Newton's rings is not however an infallible guide to a good contact with the result that printers will allow for additional vacuum time due to the mistrust factor, which again reduces the efficiency of these machines.
Other problems also occur particularly in respect of a shift in the sandwiched materials when the frame is closed causing poor registration and improper alignment of the films.